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/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package scala.actors.threadpool.locks;
import java.util.Collection;
import scala.actors.threadpool.*;
import scala.actors.threadpool.helpers.*;
/**
* A reentrant mutual exclusion {@link Lock} with the same basic
* behavior and semantics as the implicit monitor lock accessed using
* {@code synchronized} methods and statements, but with extended
* capabilities.
*
* A {@code ReentrantLock} is owned by the thread last
* successfully locking, but not yet unlocking it. A thread invoking
* {@code lock} will return, successfully acquiring the lock, when
* the lock is not owned by another thread. The method will return
* immediately if the current thread already owns the lock. This can
* be checked using methods {@link #isHeldByCurrentThread}, and {@link
* #getHoldCount}.
*
*
The constructor for this class accepts an optional
* fairness parameter. When set {@code true}, under
* contention, locks favor granting access to the longest-waiting
* thread. Otherwise this lock does not guarantee any particular
* access order. Programs using fair locks accessed by many threads
* may display lower overall throughput (i.e., are slower; often much
* slower) than those using the default setting, but have smaller
* variances in times to obtain locks and guarantee lack of
* starvation. Note however, that fairness of locks does not guarantee
* fairness of thread scheduling. Thus, one of many threads using a
* fair lock may obtain it multiple times in succession while other
* active threads are not progressing and not currently holding the
* lock.
* Also note that the untimed {@link #tryLock() tryLock} method does not
* honor the fairness setting. It will succeed if the lock
* is available even if other threads are waiting.
*
*
It is recommended practice to always immediately
* follow a call to {@code lock} with a {@code try} block, most
* typically in a before/after construction such as:
*
*
* class X {
* private final ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* lock.lock(); // block until condition holds
* try {
* // ... method body
* } finally {
* lock.unlock()
* }
* }
* }
*
*
* In addition to implementing the {@link Lock} interface, this
* class defines methods {@code isLocked} and
* {@code getLockQueueLength}, as well as some associated
* {@code protected} access methods that may be useful for
* instrumentation and monitoring.
*
*
Serialization of this class behaves in the same way as built-in
* locks: a deserialized lock is in the unlocked state, regardless of
* its state when serialized.
*
*
This lock supports a maximum of 2147483647 recursive locks by
* the same thread. Attempts to exceed this limit result in
* {@link Error} throws from locking methods.
*
* @since 1.5
* @author Doug Lea
* @author Dawid Kurzyniec
*/
public class ReentrantLock implements Lock, java.io.Serializable,
CondVar.ExclusiveLock {
private static final long serialVersionUID = 7373984872572414699L;
private final Sync sync;
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below.
*/
static abstract class Sync implements java.io.Serializable {
private static final long serialVersionUID = -5179523762034025860L;
protected transient Thread owner_ = null;
protected transient int holds_ = 0;
protected Sync() {}
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
public abstract void lock();
public abstract void lockInterruptibly() throws InterruptedException;
final void incHolds() {
int nextHolds = ++holds_;
if (nextHolds < 0)
throw new Error("Maximum lock count exceeded");
holds_ = nextHolds;
}
public boolean tryLock() {
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return true;
}
else if (caller == owner_) {
incHolds();
return true;
}
}
return false;
}
public abstract boolean tryLock(long nanos) throws InterruptedException;
public abstract void unlock();
public synchronized int getHoldCount() {
return isHeldByCurrentThread() ? holds_ : 0;
}
public synchronized boolean isHeldByCurrentThread() {
return holds_ > 0 && Thread.currentThread() == owner_;
}
public synchronized boolean isLocked() {
return owner_ != null;
}
public abstract boolean isFair();
protected synchronized Thread getOwner() {
return owner_;
}
public boolean hasQueuedThreads() {
throw new UnsupportedOperationException("Use FAIR version");
}
public int getQueueLength() {
throw new UnsupportedOperationException("Use FAIR version");
}
public Collection getQueuedThreads() {
throw new UnsupportedOperationException("Use FAIR version");
}
public boolean isQueued(Thread thread) {
throw new UnsupportedOperationException("Use FAIR version");
}
}
/**
* Sync object for non-fair locks
*/
final static class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
NonfairSync() {}
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
public void lock() {
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return;
}
else if (caller == owner_) {
incHolds();
return;
}
else {
boolean wasInterrupted = Thread.interrupted();
try {
while (true) {
try {
wait();
}
catch (InterruptedException e) {
wasInterrupted = true;
// no need to notify; if we were signalled, we
// will act as signalled, ignoring the
// interruption
}
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return;
}
}
}
finally {
if (wasInterrupted) Thread.currentThread().interrupt();
}
}
}
}
public void lockInterruptibly() throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return;
}
else if (caller == owner_) {
incHolds();
return;
}
else {
try {
do { wait(); } while (owner_ != null);
owner_ = caller;
holds_ = 1;
return;
}
catch (InterruptedException ex) {
if (owner_ == null) notify();
throw ex;
}
}
}
}
public boolean tryLock(long nanos) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return true;
}
else if (caller == owner_) {
incHolds();
return true;
}
else if (nanos <= 0)
return false;
else {
long deadline = Utils.nanoTime() + nanos;
try {
for (; ; ) {
TimeUnit.NANOSECONDS.timedWait(this, nanos);
if (caller == owner_) {
incHolds();
return true;
}
else if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return true;
}
else {
nanos = deadline - Utils.nanoTime();
if (nanos <= 0)
return false;
}
}
}
catch (InterruptedException ex) {
if (owner_ == null) notify();
throw ex;
}
}
}
}
public synchronized void unlock() {
if (Thread.currentThread() != owner_)
throw new IllegalMonitorStateException("Not owner");
if (--holds_ == 0) {
owner_ = null;
notify();
}
}
public final boolean isFair() {
return false;
}
}
/**
* Sync object for fair locks
*/
final static class FairSync extends Sync implements WaitQueue.QueuedSync {
private static final long serialVersionUID = -3000897897090466540L;
private transient WaitQueue wq_ = new FIFOWaitQueue();
FairSync() {}
public synchronized boolean recheck(WaitQueue.WaitNode node) {
Thread caller = Thread.currentThread();
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return true;
}
else if (caller == owner_) {
incHolds();
return true;
}
wq_.insert(node);
return false;
}
public synchronized void takeOver(WaitQueue.WaitNode node) {
// assert (holds_ == 1 && owner_ == Thread.currentThread()
owner_ = node.getOwner();
}
public void lock() {
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return;
}
else if (caller == owner_) {
incHolds();
return;
}
}
WaitQueue.WaitNode n = new WaitQueue.WaitNode();
n.doWaitUninterruptibly(this);
}
public void lockInterruptibly() throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return;
}
else if (caller == owner_) {
incHolds();
return;
}
}
WaitQueue.WaitNode n = new WaitQueue.WaitNode();
n.doWait(this);
}
public boolean tryLock(long nanos) throws InterruptedException {
if (Thread.interrupted()) throw new InterruptedException();
Thread caller = Thread.currentThread();
synchronized (this) {
if (owner_ == null) {
owner_ = caller;
holds_ = 1;
return true;
}
else if (caller == owner_) {
incHolds();
return true;
}
}
WaitQueue.WaitNode n = new WaitQueue.WaitNode();
return n.doTimedWait(this, nanos);
}
protected synchronized WaitQueue.WaitNode getSignallee(Thread caller) {
if (caller != owner_)
throw new IllegalMonitorStateException("Not owner");
// assert (holds_ > 0)
if (holds_ >= 2) { // current thread will keep the lock
--holds_;
return null;
}
// assert (holds_ == 1)
WaitQueue.WaitNode w = wq_.extract();
if (w == null) { // if none, clear for new arrivals
owner_ = null;
holds_ = 0;
}
return w;
}
public void unlock() {
Thread caller = Thread.currentThread();
for (;;) {
WaitQueue.WaitNode w = getSignallee(caller);
if (w == null) return; // no one to signal
if (w.signal(this)) return; // notify if still waiting, else skip
}
}
public final boolean isFair() {
return true;
}
public synchronized boolean hasQueuedThreads() {
return wq_.hasNodes();
}
public synchronized int getQueueLength() {
return wq_.getLength();
}
public synchronized Collection getQueuedThreads() {
return wq_.getWaitingThreads();
}
public synchronized boolean isQueued(Thread thread) {
return wq_.isWaiting(thread);
}
private void readObject(java.io.ObjectInputStream in)
throws java.io.IOException, ClassNotFoundException {
in.defaultReadObject();
synchronized (this) {
wq_ = new FIFOWaitQueue();
}
}
}
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantLock(boolean fair) {
sync = (fair)? (Sync)new FairSync() : new NonfairSync();
}
/**
* Acquires the lock.
*
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
*
If the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
*
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until the lock has been acquired,
* at which time the lock hold count is set to one.
*/
public void lock() {
sync.lock();
}
/**
* Acquires the lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
*
If the current thread already holds this lock then the hold count
* is incremented by one and the method returns immediately.
*
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happens:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread.
*
*
*
* If the lock is acquired by the current thread then the lock hold
* count is set to one.
*
*
If the current thread:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while acquiring
* the lock,
*
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void lockInterruptibly() throws InterruptedException {
sync.lockInterruptibly();
}
/**
* Acquires the lock only if it is not held by another thread at the time
* of invocation.
*
*
Acquires the lock if it is not held by another thread and
* returns immediately with the value {@code true}, setting the
* lock hold count to one. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()} will
* immediately acquire the lock if it is available, whether or not
* other threads are currently waiting for the lock.
* This "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting for this lock, then use
* {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
*
If the current thread already holds this lock then the hold
* count is incremented by one and the method returns {@code true}.
*
*
If the lock is held by another thread then this method will return
* immediately with the value {@code false}.
*
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} otherwise
*/
public boolean tryLock() {
return sync.tryLock();
}
/**
* Acquires the lock if it is not held by another thread within the given
* waiting time and the current thread has not been
* {@linkplain Thread#interrupt interrupted}.
*
*
Acquires the lock if it is not held by another thread and returns
* immediately with the value {@code true}, setting the lock hold count
* to one. If this lock has been set to use a fair ordering policy then
* an available lock will not be acquired if any other threads
* are waiting for the lock. This is in contrast to the {@link #tryLock()}
* method. If you want a timed {@code tryLock} that does permit barging on
* a fair lock then combine the timed and un-timed forms together:
*
*
if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
*
*
* If the current thread
* already holds this lock then the hold count is incremented by one and
* the method returns {@code true}.
*
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
*
- The specified waiting time elapses
*
*
*
* If the lock is acquired then the value {@code true} is returned and
* the lock hold count is set to one.
*
*
If the current thread:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while
* acquiring the lock,
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
*
In this implementation, as this method is an explicit
* interruption point, preference is given to responding to the
* interrupt over normal or reentrant acquisition of the lock, and
* over reporting the elapse of the waiting time.
*
* @param timeout the time to wait for the lock
* @param unit the time unit of the timeout argument
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} if the waiting time elapsed before
* the lock could be acquired
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if the time unit is null
*
*/
public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException {
return sync.tryLock(unit.toNanos(timeout));
}
/**
* Attempts to release this lock.
*
*
If the current thread is the holder of this lock then the hold
* count is decremented. If the hold count is now zero then the lock
* is released. If the current thread is not the holder of this
* lock then {@link IllegalMonitorStateException} is thrown.
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
*/
public void unlock() {
sync.unlock();
}
/**
* Returns a {@link Condition} instance for use with this
* {@link Lock} instance.
*
*
The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify notify}, and {@link
* Object#notifyAll notifyAll}) when used with the built-in
* monitor lock.
*
*
*
* - If this lock is not held when any of the {@link Condition}
* {@linkplain Condition#await() waiting} or {@linkplain
* Condition#signal signalling} methods are called, then an {@link
* IllegalMonitorStateException} is thrown.
*
*
- When the condition {@linkplain Condition#await() waiting}
* methods are called the lock is released and, before they
* return, the lock is reacquired and the lock hold count restored
* to what it was when the method was called.
*
*
- If a thread is {@linkplain Thread#interrupt interrupted}
* while waiting then the wait will terminate, an {@link
* InterruptedException} will be thrown, and the thread's
* interrupted status will be cleared.
*
*
- Waiting threads are signalled in FIFO order.
*
*
- The ordering of lock reacquisition for threads returning
* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for fair locks favors those threads that have been
* waiting the longest.
*
*
*
* @return the Condition object
*/
public Condition newCondition() {
return isFair() ? (Condition)new FIFOCondVar(this) : new CondVar(this);
}
/**
* Queries the number of holds on this lock by the current thread.
*
* A thread has a hold on a lock for each lock action that is not
* matched by an unlock action.
*
*
The hold count information is typically only used for testing and
* debugging purposes. For example, if a certain section of code should
* not be entered with the lock already held then we can assert that
* fact:
*
*
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
* public void m() {
* assert lock.getHoldCount() == 0;
* lock.lock();
* try {
* // ... method body
* } finally {
* lock.unlock();
* }
* }
* }
*
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
*/
public int getHoldCount() {
return sync.getHoldCount();
}
/**
* Queries if this lock is held by the current thread.
*
* Analogous to the {@link Thread#holdsLock} method for built-in
* monitor locks, this method is typically used for debugging and
* testing. For example, a method that should only be called while
* a lock is held can assert that this is the case:
*
*
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* assert lock.isHeldByCurrentThread();
* // ... method body
* }
* }
*
*
* It can also be used to ensure that a reentrant lock is used
* in a non-reentrant manner, for example:
*
*
* class X {
* ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* assert !lock.isHeldByCurrentThread();
* lock.lock();
* try {
* // ... method body
* } finally {
* lock.unlock();
* }
* }
* }
*
*
* @return {@code true} if current thread holds this lock and
* {@code false} otherwise
*/
public boolean isHeldByCurrentThread() {
return sync.isHeldByCurrentThread();
}
/**
* Queries if this lock is held by any thread. This method is
* designed for use in monitoring of the system state,
* not for synchronization control.
*
* @return {@code true} if any thread holds this lock and
* {@code false} otherwise
*/
public boolean isLocked() {
return sync.isLocked();
}
/**
* Returns {@code true} if this lock has fairness set true.
*
* @return {@code true} if this lock has fairness set true
*/
public final boolean isFair() {
return sync.isFair();
}
/**
* Returns the thread that currently owns this lock, or
* {@code null} if not owned. When this method is called by a
* thread that is not the owner, the return value reflects a
* best-effort approximation of current lock status. For example,
* the owner may be momentarily {@code null} even if there are
* threads trying to acquire the lock but have not yet done so.
* This method is designed to facilitate construction of
* subclasses that provide more extensive lock monitoring
* facilities.
*
* @return the owner, or {@code null} if not owned
*/
protected Thread getOwner() {
return sync.getOwner();
}
/**
* Queries whether any threads are waiting to acquire this lock. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire this lock. This method is designed primarily for use in
* monitoring of the system state.
*
* @return {@code true} if there may be other threads waiting to
* acquire the lock
*/
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
/**
* Queries whether the given thread is waiting to acquire this
* lock. Note that because cancellations may occur at any time, a
* {@code true} return does not guarantee that this thread
* will ever acquire this lock. This method is designed primarily for use
* in monitoring of the system state.
*
* @param thread the thread
* @return {@code true} if the given thread is queued waiting for this lock
* @throws NullPointerException if the thread is null
*/
public final boolean hasQueuedThread(Thread thread) {
return sync.isQueued(thread);
}
/**
* Returns an estimate of the number of threads waiting to
* acquire this lock. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring of the system state, not for synchronization
* control.
*
* @return the estimated number of threads waiting for this lock
*/
public final int getQueueLength() {
return sync.getQueueLength();
}
/**
* Returns a collection containing threads that may be waiting to
* acquire this lock. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
protected Collection getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* Queries whether any threads are waiting on the given condition
* associated with this lock. Note that because timeouts and
* interrupts may occur at any time, a {@code true} return does
* not guarantee that a future {@code signal} will awaken any
* threads. This method is designed primarily for use in
* monitoring of the system state.
*
* @param condition the condition
* @return {@code true} if there are any waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public boolean hasWaiters(Condition condition) {
return asCondVar(condition).hasWaiters();
}
/**
* Returns an estimate of the number of threads waiting on the
* given condition associated with this lock. Note that because
* timeouts and interrupts may occur at any time, the estimate
* serves only as an upper bound on the actual number of waiters.
* This method is designed for use in monitoring of the system
* state, not for synchronization control.
*
* @param condition the condition
* @return the estimated number of waiting threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
public int getWaitQueueLength(Condition condition) {
return asCondVar(condition).getWaitQueueLength();
}
/**
* Returns a collection containing those threads that may be
* waiting on the given condition associated with this lock.
* Because the actual set of threads may change dynamically while
* constructing this result, the returned collection is only a
* best-effort estimate. The elements of the returned collection
* are in no particular order. This method is designed to
* facilitate construction of subclasses that provide more
* extensive condition monitoring facilities.
*
* @param condition the condition
* @return the collection of threads
* @throws IllegalMonitorStateException if this lock is not held
* @throws IllegalArgumentException if the given condition is
* not associated with this lock
* @throws NullPointerException if the condition is null
*/
protected Collection getWaitingThreads(Condition condition) {
return asCondVar(condition).getWaitingThreads();
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes either the String {@code "Unlocked"}
* or the String {@code "Locked by"} followed by the
* {@linkplain Thread#getName name} of the owning thread.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
Thread o = getOwner();
return super.toString() + ((o == null) ?
"[Unlocked]" :
"[Locked by thread " + o.getName() + "]");
}
private CondVar asCondVar(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof CondVar))
throw new IllegalArgumentException("not owner");
CondVar condVar = (CondVar)condition;
if (condVar.lock != this)
throw new IllegalArgumentException("not owner");
return condVar;
}
}